1. Field of the Invention
The present invention relates generally to cellular communication systems, and more particularly to a method for an improved sequential searcher operation in a mobile telecommunication system supporting a variable data rate transmission.
2. Description of the Related Art
A mobile telecommunication system satisfying the IS-95 standard and standards based on IS-95 uses a searcher and rake receiver which demodulates and uses the signal energy of all paths to find a best receive path among multiple receive paths for demodulation of a reverse channel. Multiple receive paths occur as a consequence of both movement of a mobile station and changes in the wireless telecommunication environment thereby causing continuous changes in the receive-delay time. In such an environment, it is necessary to establish a path having the strongest received signal among the multiple receive paths (i.e., signals), each path having a different delay time. The searcher and rake receiver are used for this purpose.
A prior system which discloses such a method is disclosed in U.S. Pat. No. 5,644,591 entitled xe2x80x9cMethod and Apparatus for Performing Search acquisition in a CDMA Communication Systemxe2x80x9d. The ""591 patent calculates the energy level of a received signal in accordance with a pseudorandom noise (PN) sequence, selects a necessary PN sequence and decodes the received signal synchronized to the selected sequence.
A reverse channel of a mobile telecommunication system employing code division multiple access (CDMA) technology transmits walsh symbols at a rate of 4.8 kHz. Six consecutively transmitted walsh symbols make up a single power control group (PCG) with a length of 1.25 millisecond (ms) and 16 consecutive PCGs make up a frame of 20 ms. That is, a gating process divides each 20 ms frame into 16 power control groups.
In a system supporting a 9.6 Kbps rate, one frame corresponds to 192 bits (i.e. full rate) and is composed of 576 code symbols and 96 walsh symbols. Each of the 16 PCG contain 12 bits, composed of 36 code symbols and 6 walsh symbols.
Starting with the first PCG, a searcher searches for a signal at a search position corresponding to a particular delay time within a walsh symbol. The searcher may determine at the first search position whether a signal is received or not by either detecting a signal greater than a predetermined high threshold value when a signal is received, or by detecting a signal lower than a predetermined low threshold value when a signal is not received (i.e. walsh symbol). The searcher records the result of searching including delay time information and moves to the next search position.
The search position represents a delay time and the delay time means a reflection path. That is, the searcher searches for the signal of walsh symbols at a specific delay time. A plurality of search positions, for example, one hundred search positions, may exist within a walsh symbol. The base transceiver station (BTS) stores the received signals in receiving order and divides the signals into walsh symbols in accordance with time. That is, the walsh symbol is a kind of time unit and it has signal.
When each of the six walsh symbols in the PCG have been searched, the searcher calculates an average of the six search results and determines whether a signal is detected or not and then moves to the next PCG. The recorded delay time information is used to control a rake receiver.
While the search operation is capable of determining whether a signal is received or not before searching each of the six walsh symbols in the time required to receive a single PCG group (i.e., 6xc3x974.8 kHz cycles), the searcher in the prior art system does not stop the search operation at the point of determination but performs the search operation unconditionally until all six search iterations have completed, thereby wasting processing time.
A sequential searcher is constructed to avoid wasted processing time. Once the sequential searcher can determine whether a signal is received or not while searching signals at a walsh symbol/rate of 4.8 kHz, the sequential searcher moves to a first search position of the next PCG and continuously performs the search operation. Therefore, the sequential searcher can perform more search operations during the same time period.
In other words, if the sequential searcher detects the existence of a signal at search position 10 of walsh symbol 3 of PCG 0, the sequential searcher starts searching for signals, not from search position 10 of walsh symbol 4, but from search position 11 of walsh symbol 3 of new PCG 1. That is, the search position of the next PCG from the PCG where the existence of a signal is detected.
If no signal is determined to be received, the sequential search repeats the search operation at the same search position a maximum of 6 times. Here, limiting the search to a maximum of six times at the same position is intended to perform the search for a whole PCG. That is, since 6 times 4.8 kHz is equivalent to a PCG, the searcher detects the existence of a signal once each power control group. After searching six times, the search result at the corresponding searcher position is used to control a rake receiver.
Since IS-95 and standards based on the IS-95, however, support variable data rate transmission, the 16 PCGs of each 20 ms frame may or may not carry a signal depending on the data rate.
Turning now to the drawings, FIG. 1 illustrates an exemplary embodiment of a variable data rate transmission of a reverse CDMA channel as defined by IS-95B. As illustrated, since all PCGs are used in the case where a system transmits at 9600 bps, a frame transmits all 16 PCGs. On the other hand, when a system transmits at 4800 bps, the system uses half of PCGs, therefore only 8 PCGs are transmitted. Similarly, 4 PCGs are transmitted in the case of 2400 bps and 2 PCGs are transmitted in case of 1200 bps.
In the case of 4800 bps, the system selects 8 PCGs, while 4 PCGs are selected in the case of 2400 bps, and 2 PCGs are selected in the case of 1200 bps within a frame. The selection of PCGs is pseudo randomized in accordance with a data burst randomizing function. Selective transmission of PCGs appears as noise to those with no knowledge about the randomizing function of a system; by contrast, the transmission is a very orderly selection to those with knowledge of the randomizing function.
If a sequential searcher is directly applied to a system which uses a variable data rate in the aforementioned manner the sequential searcher performs the searching operation over all power control groups (PCGs) without considering whether a corresponding PCG carries a signal and may deteriorate demodulation performance. That is, it is already known by virtue of the variable data rate that certain PCGs will not carry a signal.
A non-sequential searcher, by contrast, detects a signal 6 times unconditionally at a particular position, but the sequential searcher stops searching at a current position and moves to next searcher position if the sequential searcher judges that a signal exists or not during any iteration of the six detection attempts.
That is, since the sequential searcher moves to the next searcher position of the next walsh symbol, if the sequential searcher judges the existence of a signal before the search operation is performed a maximum of 6 times, the starting position for the sequential searcher to start signal detection is not fixed. In such a case, the sequential searcher may perform the searcher operation over both PCGs carrying a signal and PCGs not carrying a signal.
For example, if the 6 iteration search operation starts from a walsh symbol located at the middle of a PCG, a sequential search performs signal searching from the rest of the walsh symbols of the PCG to a particular walsh symbol of the next PCG. Then, if the next PCG does not carry a signal in accordance with a variable data rate transmission, the sequential searcher performs searching over a noise signal and causes a problem in normally detecting a signal. Consequently, if a sequential searcher is applied to a variable data rate transmission environment, walsh symbols belonging to different PCGs are used for searching and a system""s demodulation performance is adversely affected.
It is therefore an object of the present invention to provide an improved sequential searching method to apply a sequential searcher to a system supporting a variable data rate transmission, wherein the sequential searcher operation demodulates a reverse channel in a system using code division multiple access (CDMA) technique.
Other objects and advantages of the present invention will become apparent with reference to the detailed description hereafter and attached drawings.
In accordance with one embodiment of the present invention, there is disclosed a mobile communication system supporting a variable data rate transmission and having a sequential searcher which finds an optimal path among a plurality of paths using predetermined high and low threshold values, and wherein a plurality of paths have different delay times. The method of the present invention includes dividing a time axis in increments of a walsh symbol reception rate; despreading signals received through a plurality of paths by dividing the despread signals into a plurality of power control groups (PCGs) each of which consists of a plurality of walsh symbols; selecting a PCG among the plurality of PCGs and reading a first walsh symbol among a plurality of walsh symbols belonging to the selected PCG; selecting a first search position which is located on the time axis, and calculating a correlation energy value of the first walsh symbol at the first search position, judging whether a signal is detected or not at the first search position by comparing the calculated correlation energy value of the first walsh symbol with the predetermined high threshold and the predetermined low threshold values; reading a second walsh symbol, next to the first search position on the time axis and calculating a correlation energy value of the second walsh symbol at a second search position, next to the first searcher position on time axis; if it was possible to judge whether a path is detected at the first search position, checking whether the first walsh symbol is located at the last position of the PCG on time axis; otherwise if it was impossible to judge whether a path was detected at the first search position, and if the first walsh symbol is not located at the last position of the PCG, reading the second walsh symbol and calculating a correlation energy value of the second walsh symbol at the second search position.
In accordance with another embodiment of the present invention, there is disclosed a mobile communication system supporting a variable data rate transmission and having a plurality of sequential searchers which operate in parallel to find an optimal path among a plurality of paths using predetermined high threshold and low threshold values, and wherein the plurality of paths have different delay times. A sequential path searching method comprises the steps of: despreading signals received through a plurality of paths and dividing the despreaded signals into a plurality of power control groups (PCGs) at each of the sequential searchers, each of which consists of a plurality of walsh symbols, selecting a PCG among a plurality of PCGs and reading a first walsh symbol among a plurality of walsh symbols belonging to the selected PCG, selecting a first searcher position which is located on the time axis, and calculating a correlation energy value of the first walsh symbol at the first searcher position, judging whether a path is detected or not at the first searcher position by comparing the calculated correlation energy value of the first walsh symbol with the predetermined high threshold and the predetermined low threshold values, reading a second walsh symbol, next to the first searcher position on the time axis and confirming whether a second searcher position is used by other sequential searchers, if it is possible to judge whether a path is detected at the first searcher position, wherein the second searcher position is located next to the first searcher position on the time axis divided in accordance with a walsh symbol reception rate, checking whether the first walsh symbol is located at the last position of the PCG on the time axis, if it is impossible to judge whether a path was detected at the first searcher position, reading the second walsh symbol and calculating a correlation energy value of the second walsh symbol at the first searcher position, if the first walsh symbol is not located at the last position of the PCG, reading the second walsh symbol and confirming whether the second searcher position is used by other sequential searchers, if the first walsh symbol is located at the last position of the PCG, calculating a correlation energy value of the second walsh symbol at a third searcher position, if the second searcher position is used by other sequential searchers, wherein the third searcher position is located next to the second searcher position on the time axis and calculating a correlation energy value of the second walsh symbol at the second searcher position, if the second searcher position is not used by other sequential searchers.